The present invention relates generally to solid-state lasers and more particularly to devices and methods for removing heat from a lasing medium of a solid-state laser assembly.
Many different types of solid-state lasers exist and are being used in both military and industrial applications around the world for various purposes. For example, solid-state lasers are used by the military as rangefinders to determine target distance. Solid-state lasers are also used as industrial laser welders and cutters.
Generally, a solid-state laser is a laser that uses an optical pumping source to invert population in a solid medium, which then lases via stimulated emissions. The optical pumping source may apply optical pump energy to the lasing medium so as to irradiate the lasing medium and effect a laser beam. Stated differently, the pump means provides photons that enter the lasing medium at a given wavelength A but exit from the lasing medium at a wavelength B.
There are many different lasing mediums which may be used in a solid-state laser. Indeed, lasing mediums are available in a wide variety of shapes, sizes and materials. For example, a doped YAG (yttrium-aluminum-garnet) rod may be used as the lasing host medium in the solid-state laser. YAG (Y3Al5O12) is a synthetic garnet of yttrium and aluminum oxide that is commonly used to generate laser beams in solid-state laser assemblies. The YAG crystal may be doped with neodymium (Nd), ytterbium (Yb), holmium (Ho), erbium (Er), thulium (Tm) ions, among others.
While solid-state lasers of the type described above work well and are currently being used, inefficiencies in the wavelength conversion process generate waste heat. When the lasing medium is irradiated, waste heat is generated along with the laser beam. If the waste heat is not removed, temperature gradients may build-up in the lasing medium as well as stresses and strains. In addition, the removal of the waste heat is also desirable because removing the waste heat from the lasing medium tends to improve wavelength conversion efficiency thereby resulting in improved performance of the solid-state laser.
One method currently being used to remove waste heat from lasing mediums comprises the use of streaming or flowing water over the surface of the lasing medium. Although this method will remove waste heat from a lasing medium, it does have some rather significant limitations and problems associated therewith. For example, if a large quantity of heat is to be removed from a lasing medium, a correspondingly large volume of water at a relatively high velocity must flow over the surface of the lasing medium. However, the high-volume/high-velocity flow of water over the surface of the lasing medium causes flow-induced vibrations in the lasing medium, which in turn reduce the conversion efficiency of the lasing medium. Moreover, turbulence in the high-volume/high-velocity flow of water also causes non-uniformity in the heat removal from the surface of the lasing medium, which further reduces the conversion efficiency of the lasing medium. In addition, the inlet water temperature must also be closely controlled to within +/xe2x88x922 degrees Fahrenheit, thus making temperature control of the external coolant circuit critical. Finally, although other housing components (e.g., end-walls, seal glands) may also be cooled by flowing water over them, doing so only increases the pressure drop in the cooling system and is a source of system inefficiency.
Accordingly, a need remains in the art for a device and method that is capable of removing waste heat from a lasing medium while not otherwise affecting the conversion efficiency of the lasing medium. Ideally, the device and method would remove the waste heat substantially uniformly from the lasing medium without inducing vibration or motion in the lasing medium.
In one form, the present invention provides an apparatus for removing heat from a lasing medium of a solid-state laser assembly. The apparatus comprises a working fluid and at least one condensation surface defined by a housing in which at least a portion of the lasing medium is housed. The apparatus further comprises a coolant circuit and a wick disposed to distribute the working fluid in a liquid state over at least a designated portion of an outer surface of the lasing medium and to capture working fluid condensing upon the condensation surface. During use, the wick distributes the working fluid in a liquid state over the designated portion of the outer surface of the lasing medium. During contact with the designated portion of the outer surface of the lasing medium, the working fluid evaporates and removes heat from the lasing medium. The working fluid in a vapor state contacts the condensation surface, transfers heat to the condensation surface and condenses on the condensation surface. The wick captures and returns the condensing working fluid substantially by capillary action to the lasing medium for redistribution over the designated portion of the outer surface of the lasing medium. Lastly, heat is transferred to the coolant circuit from the condensation surface.
The present invention also provides methods for removing heat from a lasing medium of a solid-state laser assembly. In one embodiment, the method comprises the steps of: using a wick to distribute a working fluid in a liquid state over at least a designated portion of an outer surface of the lasing medium, the working fluid evaporating and removing heat from the lasing medium during contact with the designated portion of the outer surface of the lasing medium, the working fluid in a vapor state contacting at least one condensation surface, the working fluid transferring heat to the condensation surface and condensing on the condensation surface; using a coolant circuit to remove heat from the condensation surface; and using the wick to capture and return the condensing working fluid to the lasing medium for redistribution over the at least a portion of the outer surface of the lasing medium.
In yet another form, the present invention provides methods for controlling the operating temperature of a lasing medium of a solid-state laser assembly. In one embodiment, the method comprises the steps of: selecting a working fluid; selecting an operating pressure for the working fluid; distributing the working fluid in a liquid state at the operating pressure substantially uniformly over at least a designated portion of an outer surface of the lasing medium, the working fluid evaporating and removing heat from the lasing medium during contact with the at least a portion of the outer surface of the lasing medium surface, the working fluid in a vapor state contacting at least one condensation surface, the working fluid transferring heat to the condensation surface and condensing on the condensation surface; removing heat from the condensation surface; and capturing and returning the condensing working fluid to the lasing medium for redistribution over the designated portion of the outer surface of the lasing medium. Accordingly, the temperature of the designated portion of the outer surface of the lasing medium is substantially equal to the boiling temperature of the working fluid at the operating pressure.
Preferably, the method of controlling the operating temperature of the lasing medium further comprises the steps of: determining the optimum operating temperature for the lasing medium; and using the optimum operating temperature for the lasing medium to select the working fluid and the operating pressure for the working fluid so that the working fluid at the operating pressure has a boiling temperature substantially equal to the optimum operating temperature for the lasing medium. Accordingly, the temperature of the designated portion of the outer surface of the lasing medium is chosen to maintain the optimum operating temperature of the lasing medium.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.